The natural band width of many ultraviolet lasers is relatively wide. For example, the spectral width of an argon fluoride excimer laser is approximately 500 pm (FWHM). One of the important uses of the ArF laser is for integrated circuit lithography in which case the beam is preferably line narrowed such that 95 percent of the beam energy is spread over a spectrum only about 1 pm.
Well known methods for line narrowing ultraviolet lasers such as excimer lasers have been well known for several years. For example, see, McKee, Terrence, Canadian Journal of Physics 1985, Vol. 63 and Laser Handbook, North-Holland Press, pages 45 and 46. These methods typically involve radially spreading the spectrum with one or more prisms or with a grating and selecting a desired spectral range by limiting the beam with an aperture. When a grating is used, the efficiency of the grating is generally increased by expanding the beam upstream of the grating and shrinking it downstream of the grating as shown in FIGS. 1A and 1B which have been extracted from the above two references.
Known method for expanding (and shrinking) the beam in a single direction include the use of one or more prisms as shown in FIG. 1A and at least two cylindrical lenses as shown in FIG. 1B.
Lenses and prisms work well as beam expanders for visible light and relatively long wavelength ultraviolet beams. However, for short wavelength ultraviolet beams, very few materials are available which are sufficiently transparent to the light. When lenses or prisms not having perfect transparency are used to line narrow a high energy beam significant heating of the optic can occur, which can produce unwanted distortions of the optical properties of the beam. This problem is expected to become more severe as photolithographic laser requirements move toward shorter wavelengths and higher output powers.
It is known that mirrors are typically less prone to produce thermal distortion than lenses and prisms, however, Applicant is not aware of any successful prior art that attempts to use mirrors for laser beam expansion for line narrowing system.
What is needed is a better beam expander for line narrowing short wavelength (&lt;200 nm) lasers which can handle high energy beams without distortion.